Wet bulb to relative humidity
Have a sling psychrometer or two thermometers? Enter the dry-bulb andwet-bulb temperatures to get the relative humidity and dew point — the classic way to measure humidity without a digital meter.
Evidence-checked. Facts and figures on this page are verified against published standards and government sources (Standard psychrometer equation (Magnus vapour pressure)). Last verified July 2026 — our methodology.
Relative humidity
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- Dew point
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- Wet-bulb depression
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- How it feels
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A wet-bulb thermometer is cooled by evaporation, so the drier the air, the bigger the gap between it and the dry-bulb reading. This tool infers the air's actual vapour pressure from that gap (the standard psychrometer equation at sea-level pressure) and compares it with the saturation pressure to give RH. Spin or fan the wet wick fully before reading for an accurate result.
Reading a psychrometer
The wet-bulb thermometer has a damp wick; as water evaporates it cools, and the difference from the dry bulb (the "depression") encodes the humidity. It is the method weather stations used for a century, and it still works when a digital hygrometer's calibration is in doubt. Once you have the RH, check whether it lands in the healthy band with thehumidity comfort checker, convert it to moisture content with theabsolute humidity calculator, or read dew point vs relative humidityfor what the numbers mean.